Systems known as Compact Disk-Read Only Memories (CD-ROM) are now well known. Such systems employ an optical-type digital disk on which a relatively large volume of information (typically in excess of 600 megabytes) may be stored.
When a disk is manufactured, it is typically optically and electronically checked to ensure that all of the data recorded on the disk can be recovered without error. However, when a disk is being played back, errors may occur. These errors may be due to, for example, dirt or scratches on the disk or irregularities in the disk's polycarbonate layer.
In order to detect and correct errors, various error detection/correction schemes have been proposed. For audio applications, Cross-interleaved Reed Solomon Code (CIRC) may be used. For CD-ROM applications, a layered error detection code/error correction code (EDC/ECC) is added to the CIRC. Although this augmented code does not correct as many errors as other types of codes, it allows for the correction of a sufficient number of errors to be useful in a CD-ROM environment. The use of both CIRC and EDC/ECC (both known also as cyclical redundancy code) are well known in the art and are described in numerous publicly available references including U.S. Pat. No. 4,937,828 to Shih et al. and U.S. Pat. No. 4,998,252 to Suzuki et al., both of which are incorporated herein by reference.
As is well known in the art, data is organized on a compact disk as a plurality of sectors. Each sector is divided into several fields, each of which serves a particular purpose.
FIG. 1A and FIG. 1B illustrate two formats for physical sectors which are commonly used in CD-ROM applications. The sector formatting shown in FIG. 1A is referred to as Yellow Book (or mode 1, type 2) format. As shown, this format provides 12 bytes of data to be used for synchronization, 4 bytes of header data, 2,048 bytes of user data, 4 bytes of EDC data, 8 blank bytes, 276 bytes of ECC data, an EDC/ECC designation field and a control field. FIG. 1B illustrates what is referred to as compact disk interactive (CD-I) (or mode 2, type 1) format. As is shown, this format provides 12 bytes of data for synchronization, 1 byte of header data, 8 bytes of subheader data, 2,048 bytes of user data, 4 bytes of EDC data, 276 bytes for ECC, 1 byte for EDC/ECC designation and 1 control byte. Yellow book and CD-I are two well known sector layouts for CD formats. However, one skilled in the art will recognize that there are numerous other ways in which sectors can be formatted. Several additional formats are already known. Other additional formats may be created at some time in the future.
As is well known, error detection and error correction are accomplished by EDC/ECC by performing appropriate algorithms on particular data bits and error detection bits in order to determine if an error has occurred. These algorithms assume that bits (and bytes) have been stored on a data medium in a particular format. Thus, for ECC/EDC to be performed properly, data bits, correction bits, and detection bits must all be in preassigned and predetermined locations. Put another way, an error detection algorithm is desirably used which corresponds to the format of the sector being read. If an algorithm for one type of sector format is used in conjunction with data stored in another type of sector format, incorrect results may be obtained.